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Will the 2013 Eagles be “healthier”? Examining Injury counts and persistence.

One of the more widely reported reasons for the 2012 Eagles’ ineptitude was the number of significant injuries the team sustained. Vick, Shady, D-Jax, and pretty much the entire Offensive Line were hit, each missing at least several games. While it must have had a negative effect on last season, it’s reasonable to believe that the injuries from last season are ALSO a reason to be optimistic this year. Surely this year’s team will not suffer as greatly. With a healthy offensive line and full seasons out of the major playmakers, the 2013 Eagles should be in a position to rebound strongly. Right?

Maybe, but caution is advised.

As is the often the case with anecdotal or out-of-context evidence, the complete data set tells a somewhat different story. To get an idea of whether we should expect the Eagles to be “less injured” this year, we need to answer two questions:

– How injured were the 2012 Eagles?

– Is there any persistence in year-to-year injuries?

How injured were the 2012 Eagles?

I’m going to use two different numbers to illustrate. The first is straightforward; it’s just the number of injury games lost by starters. Rick Gosselin of the Dallas Morning News put together this chart that shows the relevant data for every team in the league during the 2012 season. I’m going to assume his numbers are accurate.

The 2012 Eagles’ starters lost 63 games to injury last year. That sounds like a lot, and while it is a lot in absolute terms, it is good enough (bad enough?) for just 25th in the league. In other words, 7 teams lost starters to injury MORE often than the Eagles did last year.

Still, the Eagles were close to the bottom. As a result, it may be the case that we can expect the team to regress towards the league average next year, meaning fewer injuries and presumably better on-field performance. Unfortunately, to get a look at this requires more than 1 years’ data, and I don’t have/can’t find charts like the one above for previous years.

For that we need to turn back to the Football Outsider’s Adjusted Games Lost measure that I referenced two weeks ago. As I explained then, the AGL measure is a bit muddled. It uses games missed as well as the injured player’s relative importance to quantify the effects of injury. It also makes adjustments based on the injury report (Questionable, Probable, etc…) to account for the effect of a player participating at less than 100%.

As you can probably tell, there’s likely to be a lot of noise in that data. Without seeing the exact formulation, I don’t have a sense for the specific weaknesses of the stat, but we can assume it’s far from perfect.

However, the data is available for the past 5 years and I think we can assume that the statistic has been measured consistently over that time period (i.e. no changes to the formula). Therefore, we can use the stat to get an answer for both questions mentioned above.

– According to AGL, the 2012 Eagles measured 73.3 on the AGL scale. The average AGL over the past 5 seasons is 56.4.

– The standard deviation for AGL over the past 5 years is 23.1. Roughly 65% of teams over that span fall within 1 standard deviation of the average and 97% fall within 2 standard deviations, meaning the overall distribution is somewhat Normal.

What does this tell us?

Basically, it means the 2012 Eagles did suffer a relatively high number of serious injuries. However, the team’s injury count was FAR from out of the ordinary. Therefore, unlike other measures I’ve highlighted (fumbles, field position), we should NOT expect to see a big improvement to the 2013 Eagles purely as a result of mean-regression (better luck).

We’re not done yet though.

Remember that second question? The one about persistence?

Well it turns out that, for reasons I haven’t fully analyzed, Adjusted Games Lost is a surprisingly persistent statistic. That means there is a mildly strong (.30) correlation between a team’s AGL measure one year and its AGL count the following year. The most obvious reason for this would be that certain players are “injury-prone” and are therefore likely to make continuous contributions to the AGL count each season. For example, if Mike Vick is your Quarterback, you will likely get a few bumps from him each season, as opposed to someone like Eli Manning, who has now started 146 consecutive games (including playoffs). Regardless of the reasoning, the persistence tells us that since the Eagles were injury prone last year, they are somewhat likely to be injury-prone again this year.

Wrapping everything up, here’s the key takeaway:

There are a number of reasons to feel very good about the Eagles going into this year. Assuming the team will be healthier, though, is not one of them.